Pneumocystis carinii (Pc) isolated from immunosuppressed rats and from human specimens will be genotypically and phenotypically characterized. These characterized Pc isolates will be used to address the questions of source and transmission of Pc pneumonia (PcP). Chromosomes of the Pc isolates will be separated by contour clamped homogeneous electrical field (CHEF) and field inversion gel electrophoresis (FIGE). Using these techniques, we have previously shown distinct and reproducible karyotype pattern differences among Pc isolated from different colonies of rats as well as between two human isolates. In addition to karyotype patterns, Pc isolates will be characterized by hybridization of chromosome specific- and repetitive probes to blotted karyotypes and restriction enzyme cleaved Pc DNA, by antigenic profiles using a panel of monoclonal antibodies and polyclonal antisera, and by microscopic analyses to determine the developmental stages present and their DNA content (c) in the sample preparations. Stability of an individual karyotype in a rat colony and in individual patients will be monitored by sequential samplings over time. These data will determine if resident strains of Pc cause subsequent infection (reactivation of latent infection), if environmental exposure to a source of Pc leads to pneumonitis, or if both factors can contribute to PcP. Transmission of specific Pc karyotypes by close and distant routes of contact within rat colonies will be evaluated. Infection initiated by intratracheal installation of organisms of characterized Pc type will be compared to organism life cycle stages and severity of the infection found in naturally acquired PcP. The ability to initiate PcP with a characterized Pc isolate will further refine the Pc animal model and provide a rationale for establishment of a human Pc animal model. The developmental stages responsible for initiation of infection and those that remain after treatment with anti-Pc drugs will be defined. These studies will differentiate "types" of Pc for use in epidemiological studies, identify the source of Pc which leads to fulminant PcP and provide strategies for prophylactic measures and treatment regimens for the control of human PcP.